1. Field
A composite membrane, a semi-permeable separation membrane including the composite membrane, and a water treatment device including the separation membrane are disclosed.
2. Description of the Related Art
In general, a semi-permeable membrane is known to be widely used among various processes of treating a solution including various materials to exclude subject materials to be removed (e.g., salts).
Particularly, a semi-permeable membrane having high performance is required to obtain water having desired quality (e.g., drinking water, ultra-pure water) by removing salts and organic materials from the water including the salts and organic materials in a high concentration such as sea water. For example, the semi-permeable membrane may be said to have higher performance as the water permeability is higher and the salt transmittance is lower.
Generally, the structure of a semi-permeable membrane may be basically classified into a separation layer and a support layer.
The separation layer mainly acts to remove salts, and the support layer acts to support a separation layer against the water pressure including driving under a high pressure condition. Generally, since impurities are selectively separated and transmitted through a separation layer, the salt transmittance may depend upon the physical properties and structure of the separation layer. As the produced water or the inflow water is passed through the support layer, both the physical properties and structures of the separation layer and the support layer may simultaneously influence the water permeability. Among them, the physical property and structure of the support layer may have a greater influence on the water permeability than the separation layer.
The water permeability refers to how much water is transmitted through a unit area of membrane per a unit time, and a faster water passing speed is required to improve the water permeability. As the produced water or the inflow water is passed through the support layer, the support layer may act as a resistance while the water is passing. Accordingly, ideally, the support layer is required to have a low level of structural resistance.
In addition, in order to increase the hydrophilicity, the material for a support layer may have high hydrophilicity.
One of the most common methods of manufacturing a water treatment membrane is to employ non-solvent induced phase separation (NIPS), but in this case, it has limits to satisfying these two requirements.
According to the method using NIPS, a phase is separated by the miscibility difference of polymer/solvent/non-solvent to provide a pore. However, in the method, the forming of pores is determined by the diffusion speed of a solvent/non-solvent, so the pore control is not easy. In addition, as water is generally used as a non-solvent in the NIPS method, the material for a membrane may be neither dissolved nor swelled in water.
Because of this, in order to provide low structural resistance and hydrophilicity to the support layer, a material having the corresponding characteristics is needed. In order to decrease the structural resistance, the representative material having a cylindrical channel such as CNTs (carbon nanotubes) has been induced, but the hydrophobicity of CNT is a drawback. In order to provide the hydrophilicity, although titania, silica nanoparticles, or the like is introduced as inorganic particles having strong hydrophilicity, these are non-porous materials which provide high structural resistance, so they have limits in improvement of water permeability.